windows: add-second-device issues

[r10s]

i recently got the opportunity to watch someone using "add second device" from android to desktop; there are the issues that pop up:

• for whatever reason, the QR code scanner rectangle is larger than it fits on the screen, although the window was full-height. this leads to the what-to.do-hints not being visible:
  
  with just that, the user is pretty lost when starting the process from the new device. the hint "On the first device, go to "Settings / Add Second Device" and scan the code shown there" is only shown when the user discovers that there can be sth. scrolled
  on comparable large mac screens (the window screen was a normal 15~16" thinkpad laptop), the hint is shown, at least in full-height windows (it still disappears when the window is shrinked)
  it seems better to never add scrollbars there but to scale the QR code scanner rectangle and make hint being always visible (the overlay in the camera window is nice, btw)

• in contrast to mac, the camera image is not acting in the same way as a mirror. this leads to sort of intelligence test when moving the qr code around. this is a minor thing, it adds some initial confusion though, where ppl also have to deal with contrast and distance. if it gets complicated to fix that, it is maybe not worth fixing.

• "troubleshooting" is not available from the "Add as Second Device" page; adding it to "more options" menu is fine (i'd not add a dedicated button as they're comparable large and will clutter UI, usually, "troubleshooting" is not needed and ppl will find it if there is only one button left anyways)

• finally, there was https://support.delta.chat/t/add-hint-select-on-windows-desktop-private-network-to-add-a-second-device/2921/2 , but i fixed that before and i am working on a PR to get that to the "troubleshooting"

the detection of the QR code as such is pretty nice and did not cause issues

the used laptop was a standard lenovo one with a standard out-of-the-box windows using Delta Chat from Microsoft Store, nothing tweaked anywhere, all defaults